send router solicitation for ipv6 address autoconf v2

send router solicitation for ipv6 address autoconf v2

Many routers have long router advertisment interval configured by
default. The Neighbor Discovery protocol (RFC4861) has defined default
MaxRtrAdvInterval value as 600 seconds and
MinRtrAdvInterval as 0.33*MaxRtrAdvInterval. This makes
net_ipv6_autoconf fails more often than not as currently it passively
listens the RA message to perfom address autoconfiguration.

This patch tries to send router solicitation to overcome the problem of
long RA interval.

v2:
use cpu_to_be macro for network byte order conversion
add missing error handling